A Novel Non-Platinum Group Electrocatalyst for PEM Fuel Cell Application

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international journal of hydrogen energy xxx (2010) 1e8

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A novel non-platinum group electrocatalyst for PEM fuel cell application

Jin Yong Kim*, Tak-Keun Oh, Yongsoon Shin, Jeff Bonnett, K. Scott Weil

Paciﬁc Northwest National Laboratory, Richland, WA 99352, USA article info abstract

Article history: Precious-metal catalysts (predominantly Pt or Pt-based alloys supported on carbon) have Received 21 December 2009 traditionally been used to catalyze the electrode reactions in polymer electrolyte Received in revised form membrane (PEM) fuel cells. However as PEM fuel systems begin to approach commercial 19 April 2010 reality, there is an impending need to replace Pt with a lower cost alternative. The present Accepted 5 May 2010 study investigates the performance of a carbon-supported tantalum oxide material as Available online xxx a potential oxygen reduction reaction (ORR) catalyst for use on the cathode side of the PEM fuel cell membrane electrode assembly. Although bulk tantalum oxide tends to exhibit Keywords: poor electrochemical performance due to limited electrical conductivity, it displays a high Nanoscale tantaulum oxide oxygen reduction potential; one that is comparable to Pt. Analysis of the Pourbaix elec-

PEM catalyst trochemical equilibrium database also indicates that tantalum oxide (Ta2O5) is chemically Oxygen reduction stable under the pH and applied potential conditions to which the cathode catalyst is typically exposed during stack operation. Nanoscale tantalum oxide catalysts were fabricated using two approaches, by reactive oxidation sputtering and by direct chemical synthesis, each carried out on a carbon support material. Nanoscale tantalum oxide particles measuring approximately 6 nm in size that were sputtered onto carbon paper exhibited a mass-specific current density as high as one-third that of Pt when measured at 0.6 V vs. NHE. However, because of the two-dimensional nature of this particle-on-paper structure, which limits the overall length of the triple-phase boundary junctions where the oxide, carbon paper, and aqueous electrolyte meet, the corresponding area-specific current density was quite low. The second synthesis approach yielded a more extended, three- dimensional structure via chemical deposition of nanoscale tantalum oxide particles on carbon powder. These catalysts exhibited a high ORR onset potential, comparable to that of Pt, and displayed a significant improvement in the area-specific current density. Overall, the highest mass-specific current density of the carbon-powder supported catalyst wase9% of that of Pt. Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu.

1. Introduction reduction in Pt loading is still needed for large-scale auto- motive applications [1,2] and little success has been achieved While substantial progress has been made over the past in identifying suitable alternatives. At present over 30% of the decade in optimizing the anode/cathode structures and the stack cost is directly attributable to the cost of the platinum adjacent diffusion media and flow-field designs to reduce the catalysts employed in the electrodes [3], the majority of which amount of Pt needed in PEM fuel cell stacks, a five-fold is used in the cathode to catalyze the oxygen reduction

* Correspondig author. Tel.: þ1 509 376 3372. E-mail address: [email protected] (J.Y. Kim). 0360-3199/$ e see front matter Published by Elsevier Ltd on behalf of Professor T. Nejat Veziroglu. doi:10.1016/j.ijhydene.2010.05.016

Please cite this article in press as: Jin Yong Kim, et al., A novel non-platinum group electrocatalyst for PEM fuel cell application, International Journal of Hydrogen Energy (2010), doi:10.1016/j.ijhydene.2010.05.016 ARTICLE IN PRESS

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reaction (ORR) shown in Equation (1) below. This reaction is overlooked for ORR catalysis: they are not electrically known to be the rate determining step in cell operation and conductive (at least in bulk form) at low-to-moderate the cause of reduced energy efﬁciency due to a large associ- temperatures [20]. This means that while there may be cata- ated over potential [4]: lytic sites on the surface of the oxide, the ORR will be kineti-

þ cally hindered because electronic transport is limited. O þ 2H þ 2e/H O (1) 2 2 An obvious means of rectifying this problem is to mix the Any viable replacement for Pt must possess the following oxide with a chemically stable electronic conductor such as characteristics: (1) good catalytic activity for the ORR, at least graphite. More speciﬁcally, the electrochemical performance within an order of magnitude of that exhibited by Pt; (2) good of tantalum oxide can be enhanced by constructing robust electrical conductivity; (3) long-term stability under nominal triple-phase boundaries (i.e. regions where oxide, graphite, cell operating conditions in the highly acidic aqueous envi- and aqueous electrolyte meet) that effectively serve as the ronment of the stack; and (4) low cost. Arguably, the most reaction sites for catalysis. The number and length of these restrictive of these requirements is the third. Over the past catalytic regions are maximized when the tantalum oxide is several decades, the search for non-precious-metal PEMFC ﬁnely dispersed as a bonded phase on a high surface area cathode catalysts has included numerous candidates, graphitic or carbon support material. To determine the including: transition metal carbides and nitrides (e.g. WC, viability of this concept, tantalum oxide was deposited in e e Mo2N), chalcogenides (e.g. Mo4.2Ru1.8Se8 and W Co Se), and discrete, nanoscale form on a typical PEM fuel cell grade

N4-chleates (macrocycles such as iron phthalocyanine and carbon paper via reactive oxidation sputtering of a tantalum cobalt tetramethoxyphenylporphyrin) [5e10]. However, only target. Once feasibility was shown with sputtered samples, a few of these alternatives offer ORR onset potential and a more typical powder form of the catalyst was synthesized activities comparable to Pt, and those which come closest to Pt using a liquid-phase chemical synthesis approach that in terms of ORR catalytic activity fail to exhibit sufficient long- employs tantalum ethoxide as the tantalum source [21]. The term performance stability. Even the most promising and electrochemical properties of the sputtered and chemically thoroughly developed pyrolized metal porphyrins continue to synthesized carbon-supported tantalum oxide catalysts were display poor durability under practical fuel cell operating compared with a commercial Pt catalyst. conditions [11e13]. The present study investigates the performance of a carbon-supported nanoscale tantalum oxide catalyst as an 2. Experimental alternate ORR catalyst for PEM fuel cells. Although bulk tantalum oxide possesses poor electrochemical performance To demonstrate the concept of carbon-supported nanoscale due to its limited electrical conductivity, it exhibits a high tantalum oxide, tantalum oxide was deposited on 25.4 mm oxygen reduction potential (>0.95 V vs. RHE) that is compa- diameter discs of teflonized carbon paper (TGPH-090, E-TEK) rable to Pt. Stability diagrams listed in the Pourbaix electro- using reactive oxidation sputtering. In this process, the chemical equilibrium database [14] indicate that tantalum tantalum metal is DC magnetron sputtered in an atmosphere oxide (Ta2O5) is chemically stable under the pH and applied containing a partial pressure of oxygen that is sufficient to potential conditions to which the cathode catalyst is typically support in-situ oxidation during deposition. The carbon exposed during steady-state and transient fuel cell operation. substrates were mounted on a stationary water-cooled In addition, tantalum oxide is substantially lower in cost than backing plate in the sputtering chamber, which was subse- Pt, by over 2 orders of magnitude. quently evacuated to 4 10 8 Torr. The substrate surfaces 3 Ta2O5, as well as Nb2O5, received considerable attention were ion cleaned for 5 min in 0.8 10 Torr argon. Ta sput- several decades ago as potent alloying additions that enhance tering was conducted under an atmosphere of 8 mTorr of Ar/ activity/selectivity and prolong life for various oxide catalysts 10% O2 at 0.3 A d.c., 290 V, which corresponds to an approxi- employed in the selective oxidation of hydrocarbons [15]. mate deposition rate of 0.4 mm/h. It was found that discrete Their use in electrocatalysts has been more limited, with nanoscale tantalum oxide particles were deposited on the primary interest focused on their corrosion stabilizing prop- carbon support when sputtering was carried over relatively erties [16]. For example, Ta2O5 additions have shown prelim- short time periods (e1e5 min). In this way, the average particle inary synergistic improvements in carbon-supported Pt size of the deposited tantalum oxide could be controlled in the catalysts (speculated to be due to the preferential adsorption range betweene6e80 nm by adjusting both the time period for of OH groups to the oxide vs. the Pt surface) [17]. Ribeiro and each sputtering “burst” and the total number of bursts, or Andrade [18] reported a significant increase in electrode life- aggregate sputtering time. time when adding Ta2O5 to TiO2/RuO2-based catalysts used in The powder form of the catalyst was prepared using oxygen evolution reactions (carried out under conditions a chemical synthesis approach that employed tantalum eth- 3 similar to the PEMFC: 0.5 mol/dm H2SO4 at 80 C and voltage oxide [Ta(OEt)5, 99.95%; Alfa Aesar] and nanoscale carbon sweeps of 0.2e1.2 V vs. RHE). Similarly, Bonakdarpour et al. powder (average particle sizee30 nm, Asbury Carbons, Grade #: [19] found that Ta markedly reduces the dissolution of Pt in 5345R) that was functionalized with eOH groups using sputtered Pt1xTax alloy ORR catalysts by forming a thin Ta2O5 a previously reported procedure [21]. Nanoscale tantalum passivation layer. Unfortunately, the addition of Ta above w10 oxide particles form through direct reaction between the Ta at% also leads to a measurable decrease in ORR activity. The (OEt)5 and the functional groups on the surface of the carbon latter finding illustrates the key concern with these particles, a reaction that was carried out by intially dispersing compounds and the reason why they have been largely 2.0 g of the functionalized carbon in 20 ml of ethanol (99.995%;

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Table 1 e Electrochemical performance of sputtered nanoscale tantalum oxide. Sample Temperature ORR potential Current Density Current Density (C) (V) (A/cm2) @ 0.6 V vs. NHE (A/g) @ 0.6 V vs. NHE

Pt (10% Pt on 40 1.005 1.41 10 2 2.61 activated carbon) 60 0.968 1.25 10 2 2.29 80 0.975 1.28 10 2 2.36

5 4 Ta2O5 40 0.999 4.63 10 9.96 10 (Commercial 60 0.924 4.80 10 5 1.03 10 3 powder) 80 0.859 5.45 10 5 1.17 10 3

Sputtered 40 0.687 1.65 10 6 0.309 (6.5 nm 60 0.693 4.58 10 6 0.859 size) 80 0.705 5.17 10 6 0.969

Sputtered 40 0.684 1.83 10 7 0.012 (19.4 nm 60 0.693 1.48 10 6 0.093 size) 80 0.698 1.73 10 6 0.109

Sputtered 40 0.646 1.58 10 7 2.47 10 3 (77.8 nm 60 0.672 9.59 10 7 1.50 10 2 size) 80 0.695 1.51 10 6 2.36 10 2

Alfa Aesar), subsequently adding 4 ml of Ta(OEt)5, and stirring nanoscale tantalum oxide as-sputtered on carbon paper. The the resulting slurry overnight at room temperature. The Ta2O5 sample exhibits a high ORR onset potential (0.999 V vs. ethanol solvent tends to retard rapid hydrolysis and conden- NHE at 40 C and 0.859 V at 80 C), one that is comparable to the sation of Ta(OEt)5 in forming Ta2O5 because it stabilizes the Pt catalyst (1.005 V vs. NHE at 40 C and 0.975 V at 80 C). ethoxide, which affords some degree of control over Ta2O5 However, both the area-specific and mass-specific current particle size using this process. Once the reaction was densities measured at 0.6 V vs. NHE were quite low compared to complete, the ethanol was removed by evaporation at 60 C Pt, largely due to the poor electrical conductivity of Ta2O5. The under vacuum overnight. Each sample was subsequently ball- values of the area-specific current densities for the sputtered milled for 15 min in a Spex mill. samples were also small (several orders of magnitude below The electrochemical properties of the sputtered and that of Pt at comparable test conditions) because of the low chemically synthesized samples were measured and loading of active catalyst, i.e. tantalum oxide, in these samples. compared with those of high-purity Ta2O5 powder (-325 mesh, However the mass-specific current densities for the sput- 99.95%; Alfa Aesar) and a commercial carbon-supported Pt tered samples were significantly higher than those of the catalyst (10 wt% Pt on activated carbon; Alfa Aesar). The unsupported Ta2O5 powder. In fact, the sputtered sample measurements were carried out using a typical static 3-elec- consisting of the smallest tantalum oxide particles (estimated trode half-cell test arrangement that employed 0.1 N sulfuric to be 6e7 nm in diameter on average) displayed values of acid as an electrolyte. Pt mesh and a saturated Calomel elec- mass-specific reduction current density well overe1/3 that of trode were respectively used as the counter and reference the platinum catalyst at 60 and 80 C. There are two distinct electrodes. In the case of the sputtered material, the as- trends in the electrochemical data for the sputtered speci- sputtered carbon paper was employed directly as the working mens. First, there is approximately an order of magnitude electrode. When testing the powder catalyst, working elec- increase in mass-specific current density as the average oxide trodes were prepared by coating teflonized carbon (TGPH-090, particle size is reduced from 77.8 nm to 19.4 nm and again as E-TEK) discs with a paste consisting of Nafion and catalyst. the particle size is reduced from 19.4 nm to 6.5 nm. Lineal The active area of these electrodes was 1 cm2 and the loading triple-phase boundary density (the overall length of the triple- of active catalyst wase50 mg. To measure the electrochemical phase boundaries, or TPB, in the material per unit area of performance of the catalysts under simulated oxygen reduc- substrate) increases with reducing particle size according to: tion reaction conditions, cyclic voltammetry (at a scan rate of 12m 1 mV/s) was performed while slowly bubbling air through the £ ¼ (2) TPB 2r electrolyte. Testing was carried out at 40 C, 60 C, and 80 C. d A Microstructural analysis of both catalyst forms was conducted assuming that the morphology of the deposited oxide partic- by scanning electron microscopy (SEM, JEOL JSM-5900LV) and ulate can be represented as a hemispherical lens that makes transmission electron microscopy (TEM, JEOL 2010). contact with the carbon substrate along the great circle. In £ Equation (2), TPB is the lineal TPB density of the sample, m, d, and r are respectively the mass, average diameter, and density 3. Results and discussion of the particulate material deposited, and A is the surface area of the substrate on which the particulate is deposited. Listed in Table 1 are the comparative results from electro- Although this is a crude estimate of TPB density, the trend is £ chemical testing of high-purity tantalum oxide powder, accurate, indicating that TPB is inversely proportional to the a commercial carbon-supported Pt PEM fuel cell catalyst, and square of average particle size. That is, the correlation between

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current density and tantalum oxide particle size appears to nanocrystalline tantalum oxide particles are not completely support the hypothesis that an increase in TPB density should discrete, but form agglomerated clusters on the surface of the lead to greater catalytic activity. carbon paper. A corresponding energy dispersive X-ray spec- Of course the phenomenon responsible for the observed trometry analysis of the tantalum oxide particles indicates trend in current density is likely significantly more complex that the ratio of oxygen to tantalum in this material is than portrayed above. The reduction in tantalum oxide approximately 58:42 when averaged over ten separate particle size may increase catalytic activity in other ways, for measurements, which suggests that the tantalum does not example via increased oxide surface area or more energetic fully oxidized during the sputtering process (the O:Ta ratio in bonding at the surfaces of the smaller particles. However one Ta2O5 is approximately 71:29). Incomplete oxidation may be might expect that either of these factors should influence the responsible for the discrepancy in the ORR onset potentials ORR potential, a parameter that is essentially invariant of recorded for the sputtered samples vis-a`-vis those measured particle size according to the data in Table 1. The second trend for the high-purity Ta2O5 powder. that appears in this data set is the increase in current density To improve the area-specific current density of nanoscale with temperature for each of the tantalum oxide samples, tantalum oxide, we speculated that it would be necessary to including the high-purity powder. This indicates an increase extend the “two-dimensional” microstructure of the sputtered in catalytic activity with temperature, one anticipated by the heterogeneous catalyst samples into three-dimensions. This Arrhenius Law, which is currently under study in an effort to is conceptually shown in Fig. 2. In the case of the high-purity more completely understand the reaction and reaction Ta2O5 powder, the TPB density is particularly low because kinetics occurring with the tantalum oxide catalysts. these catalytic regions are formed only where the relatively Shown in Fig. 1 are typical results from TEM analysis of the large-sized powder particles contact the carbon paper. We sputtered tantalum oxide samples. In this case, the image is believe this is the key reason why the area- and mass-specific that of the sputtered sample with the smallest particle size. current densities of this material were so low. As shown The bright-field image shown in Fig. 1(a) shows the schematically in Fig. 3, the only catalytically active surface

Fig. 1 e TEM analysis results of the 6.5 nm sputtered sample: (a) bright-ﬁeld image, (b) diffraction pattern, and (c) EDS analysis conducted on representative tantalum oxide particles.

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Ta2O5 Powder: High mass specific current Low area specific current Ta2O5

TPB

Nano Ta2O5 Carbon TPB support Carbon paper

Sputtered Ta2O5 nano particles: High mass specific current Low area specific current Carbon paper

Carbon supported Ta2O5 nanoparticles: High mass specific current Carbon paper High area specific current

Fig. 2 e A schematic of nanoscale tantalum oxide/carbon composite catalysts.

regions on Ta2O5 are those within close enough proximity to are SEM micrographs collected on the resulting heterogeneous e the graphite to effect electron transfer (unlike Pt, for example, catalysts, which contain 30 77 wt% Ta2O5 as determined from in which all catalytic surface sites will be active because there wet chemical analysis. Samples containing high tantalum is no electron-transport limitation). Oxide surface that lies oxide contents, 77 wt% Ta2O5 shown in Fig. 4(a) and 63 wt% outside of these interfacial regions may be theoretically Ta2O5 shown in Fig. 4(b), appear to consist of micron-sized capable of catalyzing the ORR but is not active because of poor agglomerates of carbon particles that are almost completely electronic transport. On the other hand, sputtering of the coated by Ta2O5. In contrast, samples containing lower tantalum oxide as discrete nanosize particles on the surface of amounts of the oxide, for example the 30 wt% Ta2O5 sample the carbon increases the TPB density according to Equation (2). shown in Figs. 4(d) and 5, are composed of a finer dispersion of Therefore, the mass-specific current density can be dramati- nanoscale tantalum oxide that is attached to carbon particles cally improved, even though the area-specific current density in such a way that the carbon generally remains exposed (i.e. remains low due to the inherent limitations on oxide loading not completely covered by oxide). imposed by the two-dimensional carbon paper substrate. One The electrochemical performance of these materials is means of increasing the area-specific current density for shown in Figs. 6 and 7. To compare the results directly with nanoscale tantalum oxide is to carry out deposition on a high the commercial Pt catalyst, sample loadings of 50 mg/cm2 surface area support material. were employed in all cases. The synthesized composite cata-

Carbon particle supported nanoscale Ta2O5 media were lyst exhibits a high ORR onset potential (1.045 V vs. NHE at synthesized with varying Ta(OEt)5-to-carbon ratios using the 40 C and 0.955 V at 80 C), comparable to that of Pt as shown chemical synthesis approach described above. Shown in Fig. 4 by the example in Fig. 6. The data in Fig. 7 indicates that the

Fig. 3 e A schematic of the nanoscale carbon-support tantalum oxide catalyst system within the cathode side of the membrane electrode assembly (MEA).

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e Fig. 4 SEM micrographs of the carbon particle supported nanoscale Ta2O5 catalysts prepared with: (a) 77 wt% Ta2O5, (b)

63 wt% Ta2O5, (c) 46 wt% Ta2O5, and (d) 30 wt% Ta2O5. mass-specific current densities (measured at 0.7 V vs. NHE) percolation limit need for good electrical conductivity and increase with decreasing Ta2O5 content. The maximum thereby ensure that the TPBs are catalytically active (i.e. current density measured for this materials set was recorded electrically connected). This appears to occur for catalysts for the catalyst containing 30 wt% Ta2O5, which displayed that contain a lower amount of the Ta2O5 phase. Research is a value that was 9.3% that of the Pt catalyst at 80 C. currently underway to investigate the limits of this effect in In large part, the results in Fig. 7 can be attributed to the an effort to define the optimal tantalum oxide content, as catalyst microstructures observed in Fig. 4(aed). The well as a corresponding optimal average oxide particle size agglomerated structure of the high oxide content samples in the heterogeneous catalyst. Although the mass-specific tends to limit electrical connectivity of the nearly completely current density for the 30 wt% Ta2O5 catalyst is lower than covered carbon phase, which in turn leads to poor electrical that of the best sputtered sample (e9% vs.e30% that of Pt), the conductivity and results in the comparatively low current results demonstrate some potential of these new nanoscale densities. Conversely, catalyst structures in which the carbon catalysts and indicate possible options for further phase is exposed are more likely to meet or exceed the improvement.

30wt% Ta2O5 carbon-supported catalyst

Fig. 5 e High-magniﬁcation SEM micrograph of the carbon Fig. 6 e ORR onset potentials of a carbon particle supported particle supported nanoscale Ta2O5 catalyst prepared with nanoscale Ta2O5 catalyst (containing 30 wt% Ta2O5)

30 wt% Ta2O5. compared to a commercial Pt catalyst.

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10.0% references at 40°C at 60°C at 80°C 9.0%

8.0% [1] Mathias MF, Gasteiger HA. Fundamental research and 7.0% development challenges in polymer electrolyte fuel cells 6.0% technology. In: Proceedings of the proton conducting membrane fuel cells III symposium. Salt Lake City, UT: The 5.0% Electrochemical Society; 2004. Fall 2002. 4.0% [2] Jaffray C, Hards GA. Precious metal supply requirements. In: 3.0% Vielstich W, Lamm A, Gasteiger H, editors. Handbook of fuel cells e fundamentals, technology and applications, vol. 3. Current Density vs. Pt (%) 2.0% Chichester, UK: Wiley & Sons; 2003. p. 509. 1.0% [3] Carlson EJ, Kopf P, Sriramulu S, Yang Y. Cost analyses of fuel cell stack/systems, U.S. Department of Energy 2006 Hydrogen 0.0% 77 wt% 63 wt% 46 wt% 30 wt% Annual Report, 2006 [V.G.9]. [4] Lister S, McLean G. PEM fuel cell electrodes. J Power Sources Fig. 7 e Mass-specific current densities of a series of carbon 2004;130:61. [5] Zhong H, Zhang H, Liu G, Liang Y, Hu J, Yi B. A novel non- particle supported nanoscale Ta O catalysts measured at 2 5 noble electrocatalyst for PEM fuel cell based on molybdenum 0.7 V vs. NHE compared to a commercial Pt catalyst. nitride. Electrochem Comm 2006;8:707. [6] Ishihara A, Sibata Y, Mitsishima S, Ota K. Partially oxidized tanatalum carbonitrides as a new nonplatinum cathode for PEFC e 1. J Electrochem Soc 2008;155:B400. 4. Summary [7] Sua´rez-Alcańtara K, Solorza-Feria O. Kinetics and PEMFC performance of RuxMoySez Nanoparticles as a cathode catalyst. Electrochim Acta 2008;53:4981. Although bulk Ta O possesses poor electrochemical proper- 2 5 [8] Susac D, Sode A, Zhu L, Wong PC, Teo M, Bizzotto D, ties due to limited electrical conductivity, it displays an et al. A methodology for investigating new nonprecious oxygen reduction potential comparable to that of Pt. It was metal catalysts for PEM fuel cells. J Phys Chem B 2006; speculated that the electrochemical performance of Ta2O5 110:10762. could be improved by supporting the material on an electro- [9] Araki K, Dovidauskas S, Winnischofer H, Alexiou ADP, chemically stable conductive phase such as carbon, thereby Toma HE. A new highly efficient tetra-electronic catalyst m establishing active TPB catalytic reaction sites. To test this based on a cobalt porphyrins bound to four 3-Oxo- ruthenium acetate clusters. J. Electroanal Chem 2001;498:152. concept, carbon-paper supported nanoscale tantalum oxide [10] Chang CJ, Loh Z-H, Shi C, Anson FC, Nocera DG. Targeted catalysts were fabricated by a reactive sputtering process. proton delivery in the catalyzed reduction of oxygen to water Subsequent half-cell testing demonstrated that the resulting by bimetallic pacman porphyrins. J Am Chem Soc 2004;126: materials exhibited greater catalytic activity for the ORR than 10013.

Ta2O5 powder, with the best materials displaying mass- [11] Bezerra CWB, Zhang L, Liu H, Lee K, Marques ALB, specific current densities exceeding 1/3 that of a commercial Marques EP, et al. A review of heat-treatment effects on activity and stability of pem fuel cell catalysts for oxygen Pt catalyst under the same test conditions. However, because reduction reaction. J Power Sources 2007;173:891. the loadings in the oxide catalysts were so low, the corre- [12] Goue`recP,BiloulA,ContaminO,ScarbeckG,SavyM, sponding area-specific current densities were also low. To Riga J, et al. Oxygen reduction in acid media catalyzed improve this metric, nanoscale tantalum oxide was deposited by heat treated cobalt tetraazaannulene supported on an on high surface area carbon powder using a solution-based active charcoal: correlations between the performances chemical synthesis approach. These resulting heterogeneous after longevity tests and the active site configuration catalysts exhibited high ORR onset potential, comparable to as seen by XPS and ToF-SIMS. J Electroanal Chem 1997; 422:61. Pt, and improvement in area-specific current density. Overall, [13] Liu L, Kim H, Lee JW, Popov BN. Development of ruthenium- the highest mass-specific current densities measured in these based catalysts for oxygen reduction reaction. J Electrochem catalysts were achieved with the lowest Ta2O5-containing Soc 2007;154:A123. e samples, reaching values as high as 9% that of Pt for a Ta2O5 [14] Pourbaix M. Atlas of electrochemical equilibria in aqueous content of 30 wt%. The results demonstrate the some poten- solutions. Houston, Texas: NACE International; 1974. [15] Ushikubo T. Recent topics of research and development of tial of carbon-supported nanoscale Ta2O5 catalysts for PEM fuel cell use even though their catalytic performance needs to catalysis by niobium and tantalum oxides. Catal Today 2000; 57:331. be improved significantly more to replace Pt. [16] Cardarelli F, Taxil P, Savall A, Comninellis C, Manoli G, Leclerc O. Preparation of oxygen evolving electrodes with long service life under extreme conditions. J Appl Acknowledgments Electrochem 1998;28:245. [17] Sun Z, Chiu HC, Tseung ACC. Oxygen reduction on teflon bonded Pt/WO /C electrode in sulfuric acid. Electrochem Sol This work was supported by the Pacific Northwest National 3 St Lett 2001;4:E9. Laboratory (PNNL) lab-directed research and development [18] Ribeiro J, De Andrade AR. Characterization of RuO2-Ta2O5 (LDRD) program. PNNL is operated by Battelle Memorial coated titanium electrode microstructure, morphology, and Institute for the United States Department of Energy (U.S. electrochemical investigation. J Electrochem Soc 2004;151: DOE) under Contract DE-AC06-76RLO 1830. D106.

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[19] Bonakdarpour A, Lobel R, Sheng S, Monchesky TL, Dahn JR. pulsed dc Reactively sputtered tantalum oxide ﬁlms. J Vac Sci Acid stability and oxygen reduction activity of magnetron- Tech A 2005;23:512.

sputtered Pt1-xTax ﬁlms. J Electrochem Soc 2006;153:A2304. [21] Shin Y, Kim JY, Bonnett JF, Weil KS. Controlled Deposition of [20] Jain P, Juneja JS, Bhagwat V, Rymaszewski EJ, Toh-Ming L, Covalently Bonded Tantalum Oxide on Carbon Supports by Cale TS. Effects of substrate temperature on properties of Solvent Evaporation Solegel Process. Surf Sci, 603: 2290.